1. Field of the Invention
The present invention relates to a permanent-magnet rotary electric machine such as, for example, a stepper motor, which includes a stator having 4m (where m is an integer equal to or larger than 2) main poles as winding cores and two hybrid permanent-magnet rotors concentrically arranged with the stator and close to the stator.
2. Description of the Related Art
Rotary electric machines such as stepper motors used in office machines are required to provide a high torque within a compact size, with low vibration.
In a hybrid (hereinafter, simply referred to as “HB type”) stepper motor, a high torque can be achieved by increasing the axial thickness of a stator composed of a plurality of axially laminated plates. However, the motor diameter is determined in advance and the area of the surface of a permanent magnet of the motor from which magnetic fluxes exit is also determined. Thus, the magnetic fluxes are not increased simply by increasing the thickness of the magnet. For that reason, it is necessary to use a plurality of magnets, resulting in a structure including a plurality of HB rotors arranged in the axial direction.
More specifically, two or more rotor units are arranged in the axial direction. Each rotor unit includes a pair of magnetic rotor plates with a permanent magnet arranged therebetween. Each magnetic rotor plate has a plurality of magnetic teeth arranged at a regular pitch on its outer periphery. The rotor plates of each rotor unit are arranged with the magnetic teeth of one rotor plate offset by half a pitch from those of the other rotor plate. A spacer such as a non-magnetic disk is arranged between the rotor units. Such a multiple rotor structure, however, cannot provide a sufficiently high torque with low vibration because of the problems related to the rotor and the stator.
First, the problems related to the rotor are described. In the multiple rotor structure, the non-magnetic disk having a predetermined thickness is interposed between the rotor units for magnetic insulation, and the permanent magnets of the respective rotor units are magnetized in the same direction in the axial direction. No torque is generated at the non-magnetic disk. Moreover, the directions of interlinking magnetic fluxes from the rotor are opposite to each other around the non-magnetic disk, i.e., the boundary between two adjacent magnetic circuits, thus causing magnetic interference on the boundary between two magnetic paths. These prevent the torque from being increased. In addition, if the thickness of the non-magnetic disk is insufficient, the magnetic flux leakage occurs and makes the provided torque lower than expected. Furthermore, the use of the non-magnetic disk such as an aluminum disk increases the cost of the rotary electric machine.
The problems related to the rotor are as follows. When the axial thickness of the stator is increased to correspond to the multiple rotor structure of the rotor, the cogging torque caused by the magnetic fluxes of the permanent magnets is also increased. This cogging torque may cause a vibration torque during the motor operation or degrade the positioning accuracy.